This invention relates to soda-lime-silica glass particularly suitable for controlling transmittance of solar radiation in window glazing applications. The glass may be generally described as being green tinted, and is designed to have low heat transmittance and particularly enhanced absorption in the ultraviolet wavelength range. This is desirable for the sake of reducing the rate at which the sun's rays deteriorate plastics and fabrics in applications such as automobiles. A particular objective of the invention is to permit glass of this type to be made at a lower cost by reducing the amount of costly ingredients required.
Soda-lime-silica flat glass may be essentially characterized by the following composition on a weight percentage basis of the total glass:
______________________________________ SiO.sub.2 68-75% Na.sub.2 O 10-20 CaO 5-15 MgO 0-5 Al.sub.2 O.sub.3 0-5 K.sub.2 O 0-5 ______________________________________
Other minor ingredients, including melting and refining aids such as SO.sub.3, may also appear in the glass composition. Small amounts of K.sub.2 O, BaO or B.sub.2 O.sub.3 and other minor constituents have also sometimes been included in flat glass and may be considered optional. To this base glass are added the coloring constituents that produce the transmittance properties of the glass. The primary colorant in the category of glasses relevant to the present invention is iron, which is usually present in both the Fe.sub.2 O.sub.3 and FeO forms. As is conventional, the total amount of iron present in a glass is expressed herein as Fe.sub.2 O.sub.3, regardless of the form actually present. A typical green tinted automotive glass has about 0.5 percent by weight total iron, with the ratio of FeO to total iron being about 0.25.
Larger amounts of iron have been used in some specialty glasses intended to lower the total solar energy transmittance. However, merely increasing the amount of total iron does not reduce the ultraviolet transmittance to the extent desired even when the amount of iron present is sufficient to lower the luminous (visible light) transmittance to the minimum acceptable for automotive applications under conventional commercial melting conditions. The colorant composition and transmittance properties for examples of two commercial products of this high iron, dark green tinted type along with an example of the conventional, light green tinted glass described above are set forth below:
______________________________________ Light Green Dark Green Dark Green Example A Example B Example C ______________________________________ Total Iron (wt. %) 0.550 0.805 0.720 FeO/Tot. Iron 0.251 0.293 0.270 LT.sub.A (%) 79.5 70.9 71.6 TSUV (%) 48.1 36.9 36.5 TSIR (%) 36.7 22.9 29.2 TSET (%) 56.8 44.6 48.8 ______________________________________
The transmittance data in the table above and throughout are based on a glass thickness of 3.9 millimeters (0.154 inch). Luminous transmittance (LT.sub.A) is measured using C.I.E. standard illuminant "A" over the wavelength range 380 to 770 nanometers. Total solar ultraviolet transmittance (TSUV) is measured over the wavelength range 300 to 390 nanometers. Total solar infrared transmittance (TSIR) is measured over the wavelength range 800 to 2100 nanometers. Total solar energy transmittance (TSET) represents a computed value based on measured transmittances from 300 to 2100 nanometers at 50 nanometer intervals.
Recently, a goal has been established to limit ultraviolet transmittance to no more than 31 percent in some automotive glass. At the same time, it is a requirement that glass in vision areas of automobiles have an LT.sub.A of at least 70 percent. Simply increasing the amount of iron to lower the ultraviolet transmittance would impermissibly lower the luminous transmittance as well, so an alternative approach is needed. The use of cerium oxide in glass to reduce ultraviolet transmittance is disclosed in U.S. Pat. No. 2,860,059, and the following two examples are of commercial products that take this approach:
______________________________________ Example D Example E ______________________________________ CeO.sub.2 (wt. %) 1.06 0.70 Total Iron (wt. %) 0.780 0.858 FeO/Tot. Iron 0.290 0.282 LT.sub.A (%) 71.4 70.4 TSUV (%) 27.8 28.3 TSIR (%) 22.8 20.6 TSET (%) 44.6 42.9 ______________________________________
These glasses exhibit the desired combination of low ultraviolet transmittance and high luminous transmittance, but the high cost of cerium sources substantially increases the cost of making these glasses. It would be desirable if these objectives could be met without incurring such high raw material costs. High cerium content also produces an undesirable solarization effect, that is, the glass tends to darken upon exposure to ultraviolet radiation. For these reasons, it would be desirable to lower the amount of cerium required in this type of glass.